Hi Neeldip,
I'm not familiar with MOSAIC, but it looks like you need to extract it from the array "qsrflx" which is computed in the subroutine wetscav_cbmz_mosaic. Within module_wetscav_driver.F, note how the accumulated wet deposition is calculated for other species. You should be able to add your lines within the chem_opt=201 case, summing over all the bc aerosols. Note that you will need to add whatever you call this variable to the Registry to set the history stream.
Jordan